1. Field of the Invention
The present invention relates to a method of purifying monosilane.
2. Discussion of Related Art
Monosilane (SiH4) is a gas-phase compound which has been widely used in various applications such as semiconductors, photovoltaic cells, and thin film transistors (TFTs) for liquid crystal displays (LCDs). Recently, due to demands for greater integration and performance of electronic parts, the purity of monosilane has been considered very important.
For this reason, development of a method for yielding high-purity silicon in which the carbon content does not exceed 100 ppb while boron, arsenic, phosphorus and other electrical active compounds are present in a content less than 1 ppb, is demanded.
Usually, monosilane is prepared by diproportionating silane chlorides or by reacting a metal hydride with halogenated silane. In this process, a large amount of impurities including ethylene are produced.
In addition to ethylene, the impurities may include heavyweight impurities such as ethane, ethyl silane and diethyl silane, which are generally able to be distilled at a higher boiling point than monosilane; lightweight impurities such as methane or hydrogen, which are able to be distilled at a lower boiling point than monosilane; inorganic impurities such as boron, phosphorus and arsenic, etc.
Among the various impurities, most impurities except for ethylene may be relatively easily removed by fractional distillation. The remaining ethylene is difficult to remove by fractional distillation because it has a similar boiling point to the monosilane. Therefore, the remaining ethylene in the monosilane serves as a carbon impurity in subsequent applications thereby reducing the purity of purified monosilane.
U.S. Pat. No. 4,554,141 discloses a method of removing ethylene from monosilane, in which ethylene is removed from a gas including monosilane.
In U.S. Pat. No. 4,554,141, crystalline aluminosilicate is used as a zeolite. The porous zeolite can be used to selectively remove ethylene from monosilane, as it has excellent adsorption to the ethylene, and can be easily reused.
More particularly, U.S. Pat. No. 4,554,141 discloses a process including preferentially distilling monosilane to remove hydrocarbon compounds, passing a stream of monosilane gas through a zeolite to selectively remove ethylene, and isolating the purified monosilane.
However, according to the method of purifying monosilane, some ethylene is converted into ethyl silane while the monosilane gas passes through the zeolite. Therefore, the ethyl silane may serve as another carbon impurity, and additional purification process for ethyl silane is required.